1. Field of the Invention
The present invention relates to a signal generator for use in a video wipe generator. Such a wipe effect generator is used in a vision mixer for wiping between two video sources
2. Description of the Prior Art
Reference will now be made to FIGS. 1 to 3 of the accompanying drawings which show the background to the present invention.
FIG. 1 illustrates a known simple wipe between two video sources X and Y. As the wipe proceeds as indicated by arrow W, video X is replaced across the display by video Y (or vice versa). The effect of the wipe is achieved by mixing the video sources X and Y according to KX+(1xe2x88x92K)Y, where K is a keying signal. The keying signal is derived from a xe2x80x98solidxe2x80x99. This will be explained with reference to FIGS. 2 and 3.
A solid is an electrical signal representing a three dimensional surface of a desired shape. It may comprise at least one ramp signal and typically comprises a combination of at least two ramp signals which themselves may be modified. It may also comprise a signal defined by a polar coordinate system representing a curved surface.
FIGS. 2A illustrates a known example of a solid 125 which is a simple ramp. As shown in FIG. 2, a clip level 142 is defined. It will be appreciated that over a field or frame, the clip level 142 is defined. It will be appreciated that over a field or frame, the clip level 142 defines a plane referred to herein as the clip plane 142. The keying signal K is in known manner, derived from the solid by applying high gain to the solid and limiting the result, as shown in FIG. 2B. The keying signal has two levels 0 and 1. The transition between the levels occurs where the solid intersects the clip plane 142. The position of the intersection is varied, to produce the wipe, by adding an offset to the solid.
FIG. 3 is a schematic block diagram of a wipe generator of a vision mixer comprising a solid generator, a clip element, a gain element, a limiter and a mixer which mixes video sources X and Y in dependence upon the keying signal K. The solid generator produces a solid, for example a ramp as shown in FIG. 2A. The clip element applies an offset to the ramp to vary the intersection of the ramp with the clip plane 142 as shown in FIGS. 2A to 2C. Gain is applied to the offset ramps, in the gain element and the result limited in the limited to produce the signal K. The amount of gain applied may be varied as shown in FIG. 2B: that varies the slope of the transition between the limit values of the keying signal K.
The mixer mixes the video sources X and Y according KX+(1xe2x88x92K)Y. Thus if K=1 the output is X, if K=0 the output is Y. If the gain applied to the solid is unity and the clip offset is zero, the solid and the key signal are identical.
The example of FIGS. 1 and 2 for ease of explanation refer to a solid, a ramp, which varies as a function of only pixel position h along a line to produce a simple wipe effect. It will be appreciated that it is possible to produce solids which vary as a function of both h and v co-ordinates in a picture, where v is a line number to produce more complex wipe effects.
The present invention is concerned with the generation of a solid which allows the production of new wipe effects.
According to the present invention, there is provided a solid generator for use in a vision mixer comprising,
means for producing ramp signals within each video field or frame, each ramp signal having an amplitude which increases in a predetermined direction,
means for producing an offset signal, the offset signal having a value mV where m=1 to M where Mxe2x89xa72 mV being fixed for each set m of n ramp signal where n greater than 2 and
means for adding the ramp signal to the offset signal to produce the solid.
Such a signal generator produces a solid which allows the production of a wipe effect which is believed to be novel per se. The wipe effect comprises bands which progress across a frame as the wipe proceeds. Consecutive bands have start times which differ by controllable amount. The bands may be contiguous or spaced apart. The bands may be confined to an area of a frame, or may occupy the whole frame. The wipe may proceed in the line direction or the frame direction.